This application claims benefit of Japanese Patent Application No. 2004-213069 filed on Jul. 21, 2004, the contents of which are incorporated by the reference.
The present invention relates to step-up/down voltage choppers and, more particularly, to a step-up/down voltage chopper, which executes soft switching for reducing switching loss in operation other than that with small load and executes hard switching in the heavy load operation.
As this type of step-up/down voltage chopper, an edge resonant step-down voltage chopper is used. A resonant type circuit, on the other hand, executes pulse frequency control with a constant “on” or “off” time interval. This circuit, however, has a drawback that it is subject to efficiency deterioration in low frequency operations with low control rates. Also, for executing the resonant operation it is necessary to increase the resonant current compared to the load current in the zero current switching system, while in the zero voltage switching system the resonant voltage with respect to the DC input voltage is increased. Therefore, the circuit has a drawback that the main circuit device has a high rating compared to the circuit rating.
As a circuit for precluding this drawback, an edge resonant system has been proposed, which utilizes resonant operation only at the turn-on and -off edges of the main circuit device (see literature “The Journal of the Institute of Power Electronics”, Vol. 29, No. 1, 2004.2).
FIG. 4 shows this type of edge resonant step-down voltage chopper, and FIG. 5 shows an operation time chart of the same.
In the edge resonant circuit shown in FIG. 4, a main transistor Qs and a resonant capacitor Cr are connected in parallel, and a resonant reactor Lr is connected for resonance with an auxiliary arm constituted by an auxiliary transistor Qa and an auxiliary diode Da and the main transistor Qs.
FIG. 5 shows currents and voltages in the circuit shown in FIG. 4, i.e., base signal IBa in the auxiliary transistor Qa, resonant reactor current ILr, resonant capacitor voltage VCr, base signal IBs of the main transistor Qs, corrector current ISC of the main transistor Qs, current IDf of a flywheel diode Df, filter reactor current ILf, and corrector current IQa of the auxiliary transistor Qa.
When the main transistor Qs is“off”, the resonant capacitor Cr is charged to DC input voltage Ei. Before the main transistor Qs is turned on, the auxiliary transistor Qa is turned on by supplying IBa, thus causing discharge of the resonant capacitor Cr through the resonant reactor Lr. When the voltage Vcr across the resonant capacitor Cr becomes zero, the resonant current continuously flows through an inverse conduction diode Ds inversely parallel with the main transistor Qs. At this time, the main transistor Qs is turned on to bring about a zero voltage switching operation. This system is called ZVT (Zero Voltage Transition) system.
When the auxiliary transistor Qa is turned on after turning-on of the main transistor Qs, the resonant reactor current ILr flows through the auxiliary diode Da and the inverse conduction diode Ds to the DC input voltage side. The operation of the auxiliary transistor Qa at this time is a hard switching operation.
When the main transistor Qs is turned off, the resonant capacitor Cr connected between the two terminals of the main transistor Qs is charged from zero voltage. Zero voltage switching of the main transistor Qs is thus obtained
FIG. 6 shows auxiliary transistor base signal IBa, main transistor base signal IBs, resonant current and resonant capacitor voltage Vcr for describing the relation between the pulse width control range and the edge resonance. The edge resonant circuit utilizes a resonant phenomenon when the main transistor Qs is turned on and off. For this reason, the pulse width control of the main transistor Qs can not be obtained during the resonant operation time.
Generally, denoting the pulse width control range by T, the restriction at the turn-on time by Ton, the restriction at the turn-off time by Toff, the possible pulse width control range Tc is given asTc=T−(Ton+Toff)
The above edge resonant system, however, has the following drawbacks.                (1) Many circuit elements are involved.        (2) The control circuit is complicated.        (3) The auxiliary transistor is turned off in hard switching.        (4) The possible pulse width control range is restricted by the frequency of the resonant circuit. (The control rnage is narrow).        
Furthermore, the edge resonant step-up voltage chopper is constituted by basic elements like those of the edge resonant step-down voltage chopper. The main transistor Qs and the resonant capacitor Cr are connected in parallel, and the resonant reactor Lr is connected for resonance with the auxiliary arm constituted by the auxiliary transistor Qa and the auxiliary diode Da and the main transistor Qs.
FIG. 7 shows an example of the edge resonant step-up voltage chopper, and FIG. 8 shows the operation time chart of the same.
In the edge resonant circuit shown in FIG. 7, a main transistor Qs and a resonant capacitor Cr are connected in parallel, and a resonant reactor Lr is connected for resonance with an auxiliary arm constituted by an auxiliary transistor Qa and an auxiliary diode Da and the main transistor Qs. Detailed description in connection with FIGS. 7 and 8 is not given because it is the same as described above.
In the switching in the edge resonant circuit described above, restriction is imposed on the control width at the PWM control time. In the PWM control, duty operation from 0 to 100% is theoretically possible. In the edge resonant operation, however, the edges of the pulse switching waveform are in resonance caused by the auxiliary circuit, and it is impossible to obtain PWM control of the resonant time.
In the actual circuit operation, as in the hard switching PWM control, the edges involve non-usable times due to the on-off characteristics used in the device used for the switching. In the case of the edge resonance, however, edge resonant sections non-usable for the PWM control are present, which is about 3 to 10 times those in the hard switching case.
The edge resonance circuit has another drawback that in low PWM control range regions resonant circuit loss, generated due to edge resonance and hard switching loss generation in the switching device for the edge resonance, increases the circuit loss due to edge resonance and deteriorates the efficiency.